The term “transparent” is applied to a transmission system in which the signal remains optical without being converted into an electronic signal. Transparency in optical communication networks is a characteristic that makes it possible to reduce the cost of network equipment by eliminating optical-electrical-optical conversions, and therefore the corresponding transducers. Subsystems used in transparent WDM networks particularly include reconfigurable optical add-drop multiplexers, or ROADMs, and optical crossconnects, or OXCs. In order to construct such subsystems, equipment known as wavelength selective switches, or WSSs, are particularly attractive. This is because this equipment makes it possible to create switching nodes with any degree whatsoever having a very flexible configuration with a much simpler structure than when using discrete components, as well as a reduced size and high reliability. However, they do exhibit a relatively high cost.
US-A-2008056715 describes transparent WDM switching node architectures in which WSSs are used bidirectionally to carry out functions such as inserting optical signals, extracting optical signals, and transparently transmitting optical signals between multiple directions. Due to the bidirectional use of the WSSs, a wavelength attenuation adjustment assigns the signals to that wavelength in both directions of travel.